4. Two verbs: One action after another
When a situation includes two actions, and the second action takes place immediately after the completion of the first, the aspect particle 了 is used to indicate the completion of the first action even if the object of the verb is not quantified or does not have a modifier. These two actions may or may not be past events.

(a) The use of 就
It should be noted that 就 or another adverb is frequently used to link the two actions. Without such adverb, the sentence may not sound correct.
我爸爸每天吃了饭就吃胃药。 (吃胃药 is a habitual event. 就 is necessary)
Wǒ bàba měitiān chī le fàn jiù chī wèiyào.
My father takes acid-reducing medicine right after dinner every day.

小王跟客人说了再见就把门关上了。(把门关上了 is a past event. 就 is necessary)
Xiǎo Wáng gēn kèrén shuō le zàijiàn jiù bǎ mén guān shàng le.
Xiao Wang closed the door right after saying goodbye to his guests.

明天咱们下了课一起去看场电影，怎么样？(看电影 is a future event.)
Míngtiān zánmen xià le kè yì qǐ qù kàn chǎng diànyǐng, zěnmeyàng?
How about if we go to a movie after the class tomorrow?

(b) Modal particle 了 instead of perfective aspect particle 了
In a simple sentence, the perfective aspect particle 了 is not used if the object of the verb is not quantified or does not have a modifier; instead, modal particle 了 is used at the end of the sentence.

Mr. Wang bought a car. (This is a simple sentence.)
王先生买车了。(了 is a modal particle since it appears at the end of the sentence.)
Wáng xiānsheng mǎi chē le.
王先生买了一辆车。(了 is the perfective aspect particle since it follows the verb.)
Wáng xiānsheng mǎi le yí liàng chē.

An optional perfective aspect particle and a necessary modal particle can be used together in a simple sentence.
王先生买了车了。(=王先生买车了。)
Wáng xiānsheng mǎi le chē le.
(Incorrect: 王先生买了车。)

(Situation: Wang’s roommate Zhang asks Wang to join him for a late-night snack).
张：我要吃宵夜，你要不要吃一些？
Zhāng: Wǒ yào chī xiāoyè, nǐ yào bú yào chī yì xiē?
王：谢谢，我不吃，我刷了牙了。
Wáng: Xièxie, wǒ bù chī, wǒ shuā le yá le.
Zhang: I am going to eat a late-night snack. Would you like some?
Wang: No, thanks. I already brushed my teeth.

5. Two verbs: One indicating purpose
When a simple sentence has two verbs and the second verb indicates the purpose of the first verb, such as 来, 去 or 用, the perfective aspect particle 了 follows the second verb if it is a past event.

下午我去超级市场买了一些水果和面包。
Xiàwǔ wǒ qù chāojíshìchǎng mǎi le yì xiē shuǐguǒ hé miànbāo.
I went to the supermarket to buy some fruit and bread in the afternoon.
(Incorrect: 下午我去了超级市场买一些水果和面包。)

我用手机给我妈妈发了一个短信，告诉她我不能回家吃饭。
Wǒ yòng shǒujī gěi māma fā le yí ge duǎnxìn, gàosu tā wǒ bù néng huí jiā chī fàn.
I used a mobile phone to send my mother a text message, telling her I would not be home for dinner.
(Incorrect: 我用了手机给我妈妈发一个短信，告诉她我不能回家吃饭。)